1. Field of the Invention:
This invention relates to methods for preparing negative electrodes or anodes for use in electrical energy storage devices or batteries. More particularly, it relates to methods for preparing lithium-aluminum negative electrodes for use in such devices or batteries.
2. Description of the Prior Art:
It may be explained here that high power delivery and rapid charge, and discharge above the range of a conventional lead-acid storage battery can be obtained from a high temperature electrical energy storage battery of cell comprising a pair of electrodes, at least one of which is a negative electrode comprised of lithium and aluminum, the electrode being emersed or in contact with a fused alkali halide electrolyte. The fast charging characteristics of such a cell are mainly attributable to the highly reversible lithium-aluminum negative electrode of the cell. The positive electrode of such a cell can be carbon or any other suitable material.
The prior art methods for producing the lithium-aluminum negative electrode have been primarily either electrochemical or metallurgical.
In the electrochemical method, lithium-aluminum electrodes were produced by electrochemically charging a substantially pure aluminum electrode in an electrolyte containing lithium halide salt or salts. This electrochemical method was essentially effected by immersing a positive electrode, such as carbon, and a negative electrode, the aluminum electrode, into a molten lithium containing electrolyte and impressing an appropriate voltage across the two electrodes. Lithium in the electrolyte would diffuse into the aluminum electrode structure to form the desired lithium-aluminum electrode.
In the metallurgical method, lithium-aluminum electrodes were produced by melting a mixture containing a predetermined amount of each metal to form an alloy of lithium-aluminum.
One of the difficulties associated with the above described electrochemical method of preparing lithium-aluminum electrodes is that after they have been electrolytically formed, it is necessary to precondition the electrodes by initially operating them through a number of time consuming cycles of slow charge and discharge. If the initial cycles are carried out too quickly, regions of liquid metal alloy can be produced resulting in pitting of the electrode. Another difficulty presented in the use of electrochemically prepared lithium-aluminum electrodes is their lack of dimensional stability due to the fact that during the forming and preconditioning steps, they have been found to sometimes expand.
The preparation of lithium-aluminum electrodes by metallurgical techniques has also presented problems in that it has been difficult in the past to obtain lithium-aluminum alloys with compositions much in excess of 5 weight percent lithium with acceptable purity levels and lack of fragility. Due to this less than desirable percentage of lithium, it was necessary to place the 5 weight percent lithium-aluminum electrodes in a molten salt formation tank and electrolytically "pump-up" the electrode with lithium from the electrolyte, to lithium percentages of at least about 12%, with the preferred range being about 6 to 25 weight percent lithium. Manufacturers of lithium alloys have attempted to fabricate lithium-aluminum electrodes having the preferred range of weight percent lithium by melting the components together in low humidity, dry, and argon atmospheres, and then pouring the molten liquid into molds, but they have not been entirely successful for the reasons that the handling of such a molten liquid is extremely hazardous and unusually in an inert atmosphere. At the elevated temperatures required to melt the components, the lithium acts as a getter for oxygen, nitrogen and water vapor. The resulting alloy by these procedures is generally brittle, contaminated and in the form of thick slabs that are difficult and literally impossible to utilize for battery manufacture.
Another problem associated with metallurgically prepared lithium-aluminum electrodes is that of providing low impurity levels while casting, and after crushing, remelting of the metals or compacting the powder by powder metallurgical methods to form the lithium-aluminum alloy. High inpurity levels in the lithium-aluminum alloy required that the fabricated electrodes be cleaned by electrolytic methods similar to those described above for the preparation of electrodes by electrolytic methods.
Therefore, in view of the above, it is a primary object of the present invention to provide a method of preparing negative electrodes comprised of a lithium-aluminum alloy relatively free of impurities for use in high-temperature electrochemical cells.
Another object of the present invention is to provide a method of preparing solid lithium-aluminum electrodes comprised of from about 6 to 25 weight percent, based on total composition, lithium without employing lengthy electrochemical processes or the necessity of handling or working with molten metals as in the case of prior art metallurgical methods of preparing such electrodes.
Still another object of the present invention, is to provide essentially uniform and dimensionally stable electrodes comprised of lithium in amounts of from about 6 to 25 weight percent, based on total composition, and from about 75 to 94 weight percent, based on total composition, aluminum.
The foregoing and other objects and features of the invention will be evident from the following detailed description thereof.